Ethan R. Siegel

Towards a precision cosmology from starburst galaxies at z > 2

This work investigates the use of a well-known empirical correlation between the velocity dispersion, metallicity and luminosity in Hβ of nearby H II galaxies to measure the distances to H II-like starburst galaxies at high redshifts. This correlation is applied to a sample of 15 starburst galaxies with redshifts between z = 2.17 and z = 3.39 to constrain Ω m , using data available from the literature. A best-fitting value of Ω m = 0.21 +0.30 -0.12 in a A-dominated universe and of Ω m = 0.11 +0.37 -0.19 in an open universe is obtained. A detailed analysis of systematic errors, their causes and their effects on the values derived for the distance moduli and Ω m is carried out. A discussion of how future work will improve constraints on Ω m by reducing the errors is also presented.

THE EFFECTS OF INHOMOGENEITIES ON COSMIC EXPANSION

We evaluate the effect of inhomogeneity energy on the expansion rate of the universe. Our method is to expand to Newtonian order in potential and velocity but to take into account fully nonlinear density inhomogeneities. To linear order in density, the kinetic and gravitational potential energies contribute to the total energy of the universe with the same scaling with expansion factor as spatial curvature. In the strongly nonlinear regime, growth saturates, and the net effect of the inhomogeneity energy on the expansion rate remains negligible at all times. In particular, the contributions due to inhomogeneities never mimic the effects of dark energy or induce an accelerated expansion.

Cosmological Structure Formation Creates Large-Scale Magnetic Fields

This paper examines the generation of seed magnetic fields due to the growth of cosmological perturbations. In the radiation era, different rates of scattering from photons induce local differences in the ion and electron density and velocity fields. The currents due to the relative motion of these fluids generate magnetic fields on all cosmological scales, peaking at a magnitude of O(10-24 G) at the epoch of recombination. Magnetic fields generated in this manner provide a promising candidate for the seeds of magnetic fields currently observed on galactic and extragalactic scales.

Probing dark matter substructure with pulsar timing

We demonstrate that pulsar timing measurements may potentially be able to detect the presence of dark matter substructure within our own Galaxy. As dark matter substructure transits near the line of sight between a pulsar and an observer, the change in the gravitational field will result in a delay of the light travel-time of photons. We calculate the effect of this delay due to transiting dark matter substructure and find that the effect on pulsar timing ought to be observable over decadal time-scales for a wide range of substructure masses and density profiles. We find that transiting dark matter substructure with masses above 10 -2 M ⊙ ought to be detectable at present by these means. With improved measurements, this method may be able to distinguish between baryonic, thermal non-baryonic, and non-thermal non-baryonic types of dark matter. Additionally, information about structure formation on small scales and the density profiles of Galactic dark matter substructure can be extracted via this method.

A thermal graviton background from extra dimensions

Inflationary cosmology predicts a low-amplitude graviton background across a wide range of frequencies. This Letter shows that if one or more extra dimensions exist, the graviton background may have a thermal spectrum instead, dependent on the fundamental scale of the extra dimensions. The energy density is shown to be significant enough that it can affect nucleosynthesis in a substantial way. The possibility of direct detection of a thermal graviton background using the 21-cm hydrogen line is discussed. Alternative explanations for the creation of a thermal graviton background are also examined.

Dark matter on the smallest scales

Abstract This work investigates the dark matters structures that form on the smallest cosmological scales. We find that the types and abundances of structures which form at approximately Earth-mass scales are very sensitive to the nature of dark matter. We explore various candidates for dark matter and determine the corresponding properties of small-scale structure. In particular, we discuss possibilities for indirect detection of dark matter through small-scale structure, and comment on the potential of these methods for discriminating between dark matter candidates.

Dark Matter in Minimal Trinification

This chapter discusses the dark matter in minimal trinification. The lightest of the neutral singlets in the minimal trinification model is overabundant at freeze out with the consequence that it is completely ruled out by cosmological data. It is overcome if the particle is not very stable but then it ruins Big-Bang nucleosynthesis (BBN) and creates another big problem. Because of the abundant number of scalar fields, it is possible that baryogenesis is achieved at GUT scales or at electroweak scales through a first order phase transition. The GUT model is not a complete model of particle physics as it does not withstand one of the most needed cosmological requirements: the existence of Dark Matter. The chapter outlines the trinification model, lists its salient features and focuses on its advantages and downsides. The chapter discusses possible improvements of the model that would circumvent its difficulty in providing a viable DM candidate.